1. Field of the Invention
The present invention relates to a phosphate-based dental investing material used for molding a dental mold, and the material is used for a mold for casting a dental metal product such as a crown or the like so as to cast the metal product having a smooth surface.
2. Description of the Conventional Art
As for a dental investing material, a gypsum-based investing material and a phosphate-based investing material have been known. The gypsum-based investing material is preferably used since an investing operation and a digging operation of a cast product are easy. However, gypsum is decomposed at 700 degree C. or more under the existence of carbon contained in a wax or the like, and thus cannot be used for casting an alloy having a high melting point such as an alloy for porcelain fused to metal. Therefore, when an alloy having a high melting point is cast, a phosphate-based investing material has been generally used. Further, in order to save the time necessary for the preheating of a mold and the incineration of a wax pattern, quick heating has been widely used in recent years. In the quick heating, an investing material, in which a wax pattern is invested, is directly taken into a furnace at a temperature of 800 to 900 degree C. together with a cast ring.
As the phosphate-based investing material capable of quickly heating, Japanese Patent Application Laid Open No. 06(1994)-336409 discloses an investing material including a refractory material and a phosphate-based binder (for example, a mixing material of magnesia clinker and ammonium primary phosphate), where tridymite is added to quartz and/or cristobalite used as the refractory material. As for the investing material indicated in this patent document, a part of a quartz and/or cristobalite having excellent properties to compensate casting contraction of metal is replaced by tridymite having comparatively low thermal expansion coefficient and no quick increase of the thermal expansion coefficient due to temperature increasing. Thus, crack does not occur even when quickly heating the material after investing, and the time necessary for the preheating of a mold and the incineration of a wax pattern can be largely shortened.
However, when a mold strength increases by decreasing the mixing ratio of a liquid in order to prevent the crack or surface peeling of a mold, flowability of an investing material decreases, and thus the investing operation becomes hard. Further, when a refractory material having a large particle diameter is used as an aggregate powder so as to improve the flowability, there is a problem that a cast surface is roughed. When the particle diameter of the aggregate powder is made small, the flowability decreases, so that the aggregate powder coagulates so as to become a lump having a larger diameter than that of the refractory material powder. Thus, the surface roughness of a cast product increases so that it is necessary to polish the surface of the cast product in order to decrease the surface roughness. However, polishing takes a long time, and it is uneconomical when casting a noble-metals product especially. Furthermore, the refractory material is restricted in quartz, cristobalite, and tridymite.